There are a number of enhanced reflective display media which offer numerous benefits such as enhanced optical appearance, the ability to be constructed in large form factors, capable of being formed using flexible substrates, characterized by easy manufacturability and manufactured at low cost. Such reflective display media include microencapsulated electrophoretic displays, rotating ball displays, suspended particle displays, and composites of liquid crystals with polymers (known by many names including but not limited to polymer dispersed liquid crystals, polymer stabilized liquid crystals, and liquid crystal gels). Electrophoretic display media, generally characterized by the movement of particles through an applied electric field, are highly reflective, can be made bistable, and consume very little power. Further, encapsulated electrophoretic displays also may be printed. These properties allow encapsulated electrophoretic display media to be used in many applications for which traditional electronic displays are not suitable, such as flexible, printed displays.
While bichromatic electrophoretic displays have been demonstrated in a limited range of colors (e.g. black/white or yellow/red), to date there has not been successful commercialization of a full-color electrophoretic display. Indeed, no reflective display technology to date has shown itself capable of satisfactory color. Full-color reflective displays typically are deficient when compared to emissive displays in at least two important areas: brightness and color saturation.
One traditional technique for achieving a bright, full-color display which is known in the art of emissive displays is to create sub-pixels that are red, green and blue. In this system, each pixel has two states: on, or the emission of color; and off. Since light blends from these sub-pixels, the overall pixel can take on a variety of colors and color combinations. In an emissive display, the visual result is the summation of the wavelengths emitted by the sub-pixels at selected intensities, white is seen when red, green and blue are all active in balanced proportion or full intensity. The brightness of the white image is controlled by the intensities of emission of light by the sub-pixels. Black is seen when none are active or, equivalently, when all are emitting at zero intensity. As an additional example, a red visual display appears when the red sub-pixel is active while the green and blue are inactive, and thus only red light is emitted.
It is known that this method can be applied to bichromatic reflective displays, typically using the cyan-magenta-yellow subtractive color system. In this system, the reflective sub-pixels absorb characteristic portions of the optical spectrum, rather than generating characteristic portions of the spectrum as do the pixels in an emissive display. White reflects everything, or equivalently absorbs nothing. A colored reflective material reflects light corresponding in wavelength to the color seen, and absorbs the remainder of the wavelengths in the visible spectrum. To achieve a black display, all three sub-pixels are turned on, and they absorb complementary portions of the spectrum.
However, the colors displayed by a full-color display as described above are sub-optimal. For example, to display red, one pixel displays magenta, one displays yellow, and one displays white. The white sub-pixel reduces the saturation of red in the image and reduces display contrast The overall effect is a washed out red. This further illustrates why no method to date has been capable of generating a high-contrast, high-brightness full color reflective display with good color saturation.